1. Field of the Invention
The present invention relates to an electronic still camera, and in particular, to a digital electronic still camera in which a video signal representing a still image is stored in the form of digital data.
2. Description of the Related Art
In an example, an electronic still camera described in the Japanese Patent Laid-Open Publication No. 183592/1984 includes a memory detachably connectable to the body of the camera having an imaging optical system and a solid-state imaging device such that a video signal representing a still picture produced by the imaging device is stored in the form of a digital signal in the memory. The memory loaded with the video signal may be removed from the electronic still camera and may then be installed in a playback apparatus such that the video signal read out of the memory is reproduced in the form of a visible image on a screen of a video monitor.
In usual cases, a difficulty arises in appropriately using the video signal produced from a color solid-state imaging device in the original form. More specifically, video signal processing is required to achieve, for example, a gradation correction such as a gamma, .gamma., correction and a color adjustment such as a white balance adjustment. In the conventional electronic still camera system, such video signal processing has been accomplished in a playback apparatus.
It has been well known that the video signal produced from the solid-state image pickup device is an analog signal. Conventionally, in a digital still camera, the correction of the .gamma. characteristic and the white balance adjustment develop a digital signal into which the video signal attained from the imaging device is converted. For details, reference is made to, for example. Ohnishi, et al, "Fully Digitized Electronic Still Camera", the Journal of The Institute of Television Engineers of Japan, Vol. 37, No. 10, pp. 863-868 (1983) and U.S. Pat. No. 4,914,746 to Nishi et al.
In an ordinary analog-to-digital conversion, for the input signals, the size of the quantization step does not vary between the levels thereof, namely, the same step size is adopted for the low-level and high-level signals. In addition, the .gamma. correction is achieved so as to be emphasized on the low-level signal as compared with the high-level signal. As a consequence, when the .gamma. correction is conducted to a digital signal into which a signal is converted, the quantization error may excessively be emphasized for the signal having a small amplitude, and hence the played back picture is produced with coarse gradation levels.
In the solid-state imaging device, the sensitivity to an incident light of each cell constituting a photosensitive cell array thereof varies depending on the separated colors of filter segments from a color filter. In order to remove the difference in sensitivity, the white balance adjustment is conducted on the video signal. In a video signal obtained from an imaging or photosensitive cell associated with a color segment having a low transimissivity to the incident light, for example, the blue segment, the signal level is relatively lower than the level of signals associated with the remaining segments. As a consequence, when converting such a low-level signal into a digital signal by an analog-to-digital converter, the dynamic range thereof cannot be effectively utilized, and hence, even if the white balance adjustment is achieved thereafter, the signal-to-noise ratio, S/N, thereof is lowered as a result.